A method of providing a surface of a steel substrate with an aluminum coating

ABSTRACT

The invention concerns a method of providing a surface of a steel substrate with an aluminum coating consisting essentially of the steps of electrolytically coating the said surface with a layer of iron, wetting the iron layer with a film of an aqueous solution consisting essentially of water and an alkali metal silicate, coating the said wetted layer of iron with a layer of aluminum powder which forms an alloy with the steel substrate when heat treated at a temperature of 502* - 600* C, compacting the said layers to the surface of the steel substrate at a temperature below the sintering temperature whereby the particles of aluminum powder in the compacted layer thereof are compacted to each other and to the steel substrate without sintering occurring, and heat treating the compacted steel substrate at a temperature of 502* - 600* C over a period of time whereby sintering and bonding of the particles to each other and to the steel substrate occurs, the said iron layer serving to control nucleation of said alloy during the heat treatment and having a thickness varying from a minimum of about 0.5 micron when the heat treatment is at 502* C to a minimum of about 3.5 microns when the heat treatment is at 600* C.

ilited States Patent [191 Jackson et al.

[ METHOD OF PROVIDING A SURFACE OF A STEEL SUBSTRATE WITH AN ALUMINUM COATING [75] Inventors: Albert Edward Jackson,

Gwernaffield Mold; Ernest Wynne Williams, Mold, both of England [73] Assignee: British Steel Corporation, London,

' England [22] Filed: Mar. 27, 1972 [2]] Appl. No.: 238,287

Related US. Application Data [63] Continuation-impart of Ser. No. 814,145, April 7,

1969, abandoned.

[52] US. Cl 204/34, 204/37 R, 204/38 S, 204/48, 117/105, 75/208 CS [51] Int. Cl. C23b 5/52, C23b 5/04, C23c 7/00 [58] Field of Search 204/38 R, 38 S, 48, 204/37 R, 34; 117/105; 75/208 CS Aug. 28, 1973 Primary Examiner-John l-l. Mack Assistant Examiner-R. L. Andrews Att0rneyShanley & ONeil [57] ABSTRACT The invention concerns a method of providing a surface ofa steel substrate with an aluminum coating consisting essentially of the steps of electrolytically coating the said surface with a layer of iron, wetting the iron layer with a film of an aqueous solution consisting essentially of water and an alkali metal silicate, coating the said wetted layer of iron with a layer of aluminum powder which forms an. alloy with the steel substrate when heat treated at a temperature of 502 600 C, compacting the said layers tothe surface of the steel substrate at a temperature below the sintering temperature whereby the particles of aluminum powder in the compacted layer thereof are compacted to each other and to the steel substrate without sintering occurring, and heat treating the compacted steel substrate at a temperature of 502 600 C over a period of time whereby sintering and bonding of the particles to each other and to the steel substrate occurs, the said iron layer serving to control nucleation of said alloy during the heat treatment and having a thickness varying from a minimum of about 0.5 micron when the heat treatment is at 502 C to a minimum of about 3.5 microns when the heat treatment is at 600 C.

19 Claims, 3 Drawing Figures Patented Aug. 28, 1973 2 Sheets-Sheet 1 Patented Aug. 28, 1973 2 Sheets-Sheet 2 a Q Q R Q Q a h l l h M R mw Q Q% g E 8 mw Q a hm Qm m QM A METHOD OF PROVIDING A SURFACE OF A STEEL SUBSTRATE WITH AN ALUMINUM COATING RELATED APPLICATION This application is a continuation-in-part of U5. application Ser. No. 814,145 filed Apr. 7, 1969 now abandoned by Albert Edward Jackson and Ernest Wynne Williams, for METHOD OF PROVIDING A METALLIC COATING ON A METALLIC SUBSTRATE.

BACKGROUND OF THE INVENTION This invention concerns a method of providing a surface of a steel substrate with an aluminum coating.

One of the outstanding properties of aluminum coated steel is its oxidation resistance at elevated temperatures. At temperatures below about 500 C the product remains bright and lustrous combining the well-known oxidation resistance of pure aluminum with the strength of steel. Above about 600 C the aluminum coating alloys with the steel substrate by diffusion, yielding a dark grey surface which again has oxidation resisting properties. Between these two temperatures diffusion occurs slowly and the aluminum coating is separated from the steel substrate by an intermediate layer of alloy, the thickness of which depends upon the temperature and the length of time for which the aluminum coated steel is subjected to this temperature. Whilst the presence of this intermediate layer is normally not detrimental during service, it is undesirable in the product prior to fabrication because its brittle nature causes the aluminum coating to crack and even delaminate in forming operations.

Moreover, if the product is used for long periods at a temperature in the range 500-600 C, the aluminum coating can blister and in some cases become detached from the steel substrate. This is because, in the initial stages of alloying, the adhesion between the aluminum coating and the thin alloy layer is of a very low order, and thermal cycling can cause microseparation leading to delamination.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l-a and FIG. l-b illustrate one presently preferred arrangement of coating apparatus for use in practicing one variant of the invention, FIG. 1-.b being a continuation of FIG. l-a; and

FIG. 2 illustrates another presently preferred arrangement of coating apparatus suitable for use in practicing another variant of the invention.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED VARIANTS THEREOF According therefore to the present invention there is provided a method of providing a surface of a steel substrate with an aluminum coating consisting essentially of the steps of electrolytically coating the said surface with a layer of iron, wetting the iron layer with a film of an aqueous solution consisting essentially of water and an alkali metal silicate, coating the said wetted layer of iron with a. layer of aluminum powder 'which forms an alloy with the steel substrate when heat treated at a temperature of 502 600 C, compacting the said layers to the surface of the steel substrate at a temperature below the sintering temperature whereby the particles of aluminum powder in the compacted layer thereof are compacted to each other and to the steel substrate without sintering occurring, and heat treating the compacted steel substrate at a temperature of 502 600 C over a period of time whereby sintering and bonding of the particles to each other and to the steel substrate occurs, the said iron layer serving to control nucleation of said alloy during the heat treatment and having a thickness varying from a minimum of about 0.5 micron when the heat treatment is at 502 C to a minimum of about 3.5 microns when the heat treatment'is at 600 C.

The relatively soft nature of the electrolytically coated layer of iron enables the aluminum particles to be effectively embedded therein. In contrast thereto, if an attempt were made to compact the aluminum powder into a pickled surface of the steel substrate itself, the powder could not be satisfactorily embedded in the pickled surface because of the relatively smooth hard nature of the latter. The ability of the aluminum particles to be efiectively embedded in the electrolytically coated layer of iron also arises in part from the fact that, on a micro-scale, the electro-plated surface is rougher than a pickled surface, since pickling tends to attack preferentially the microscopic high spots on a steel substrate with the result that a pickled surface is smooth.

The provision of the electrolytically coated layer of iron, moreover, enables one to use a much larger quantity of the alkali metal siiicate solution than would otherwise be possible. This is because the alloying mechanism which occurs at high temperatures between the steel substrate and the aluminum powder can operate effectively only when the level of impurities in the steel substrate is not too high; However, the surface of the steel substrate itself is relatively speaking impure by reason of the carbon and the other additions to the steel. Consequently, the amount of additional impurity, constituted by the alkali metal silicate solution, which can be tolerated if the electrolytically coated layer of iron is not-provided,is very low. However, in the case of the present invention, the electrolytically coated layer of iron is of course free of impurities, and consequently a large amount of the alkali metal silicate solution can be tolerated. This in turn means that there is a consequent increase in the adhesion of the aluminium powder to the steel substrate before heat treatment, and this in turn enables high speed rolling to be effected, whereby the rate of production is substantially increased.

' The heat treatment is preferably in the range of 502 to 575 C and is best operated in the range of 502 to 525 C. The thickness of the iron layer is, moreover, preferably at least 1.5 microns or, better, at least 2microns.

The compacted steel substrate may be heat treated for about 15 to 20 hours. I

The aluminum powdered particles may be applied to the wetted iron layer in an electrolytically charged condition.

The substrate is preferably a moving steel strip each of whose opposite sides is provided with an aluminum coating. 7

The steel substrate may be subjected to pickling in an acid, the electrolyte employed during the said electrolytic coating being derived from liquor resulting from the said'pickling. Any remaining acid in the pickle liquor may be neutralised with scrap iron, and the pickle liquor may be diluted with water prior to its being used as the electrolyte.

The steel substrate, immediately after receiving the electrolytic coating, may be dried so as to solidify any electrolyte on the steel substrate, the solidified electrolyte being thereafter removed by washing with water, and the steel substrate being then wetted with the said aqueous solution of an alkali metal silicate. Preferably drying is effected by directing air onto the steel sub strate.

The steel strip may be passed, with a predetermined surface thereof lowermost, through at least one first plating bath, and may then be passed, with the opposite surface thereof lowennost, through at least one second plating bath, each of the first and second plating baths being provided with an iron anode above which the steel strip is passed, and the said at least one second plating bath being provided with an auxiliary anode disposed above the steel strip, the auxiliary anode being insoluble in the electrolyte in the second plating bath and being operated at a low voltage which cathodically polarises the steel strip and ensures that the latter is not pickled by the said electrolyte. The auxiliary anode may be a titanium anode and may be operated at a current density of about 5 amperes per square foot.

The electrolyte employed in the electrolytic coating should desirably have a ferrous iron content of at least 70 grams per litre, since failure to maintain this concentration can in certain cases cause blisters and poor adhesion of the aluminum coating to the steel substrate. Such failure can also occur unless there is the correct relationship between the temperature and the pH of the electrolyte employed in the electrolytic coating. The plating solution may, for example, operate naturally at a pH of 1.2, and at this pH the temperature must be at least 70 C. Higher pH values .can be obtained by constant additions of alkali to the plating solution, in which case lower temperatures can be tolerated. However, it is preferred to use the natural pH of the system and to this end a temperature of 75 C is preferred.

Immediately prior to the electrolytic coating, the steel substrate is preferably wetted with a liquid whose pH is substantially the same as that of the electrolyte employed in the electrolytic coating. Thus the electrolyte itself may be employed as the said liquid.

The aluminum coated steel substrate may be cold reduced, e.g., by 60 percent or more, and then annealed at a temperature within the range 500 to 525 C.

Compaction may be effected by passing the coated substrate between a pair of rollers, the alkali metal silicate solution reducing the extent to which the rollers dislodge the aluminum powder.

The invention is illustrated by way of example only in the following Examples, the accompanying FIGS. l-a and l-b illustrating the apparatus used in Example 1, and the accompanying FIG. 2 illustrating the apparatus used in Example ll.

EXAMPLE l A mild steel strip 7, to be coated, having opposite surfaces 8, 9, is wound from a roll thereof and passes at a speed of up to 300 feet per minute through two degreasing baths 12 where the strip 7 is degreased by a conventional degreasing liquid. It is then rinsed in cold water at a wash station 14, whereafter it is pickled in two pickling baths 16, so that all the surface area of the strip is cleaned by having any metallic oxide or other contamination removed therefrom.

The pickling baths 16 may if desired contain dilute nitric acid, e.g., they may contain five percent by weight nitric acid and in addition may contain 25 grams per litre of urea. The latter inhibits the accumulation in the bath of nitrous acid and oxides of nitrogen which, if allowed to accumulate, would render the bath inactive. The use of a nitric acid bath effects a pickle which does not introduce hydrogen into the surface of the strip 7 and this may be of importance since the expulsion of the hydrogen during heating can in some cases render the aluminum coating, which is subsequently applied to the strip 7, liable to delamination.

More usually, however, the pickling baths 16 contain sulphuric or hydrochloric acid, since the coating of the steel substrate with the electroplated layer of iron described hereafter has the effect of reducing the hydrogen problem referred to above.

The strip 7 is then rinsed in cold water at a further wash station 18, whereafter it passes to the first of two electrolytic plating baths 20, where a first layer of iron is applied to the strip.

The electroplating baths 20 contain an aqueous iron salt solution, such as ferrous chloride or ferrous sulphate. When the pickling baths 16 contain hydrochloric acid or sulphuric acid, then the waste pickle liquor therefrom may be employed in the electroplating baths 20.

Thus for example at the present time most hot rolled steel coil is pickled in hydrochloric acid and this means that considerable quantities of ferrous chloride are available in the waste pickle liquor. The term waste 'is used to describe the pickle liquor when all the useful acid therein has been used up by virtue of its reacting with the steel, and the liquor is really a concentrated solution of ferrous chloride. It is possible to recover the hydrochloric acid by decomposing the ferrous chloride, in which case the hydrochloric acid can then be re-used for further pickling. Alternatively, as indicated above, the waste pickle liquor can be used as the electrolyte for the iron electro-deposition. This can be done merely by neutralising any remaining acid in the waste pickle liquor by means of scrap iron, and then diluting the resultant concentrated ferrous chloride solution with water so as to bring it to a suitable strength, which may for example be grams per litre of iron in the ferrous state.

From the electroplating baths 20, the strip 7 is rinsed at a wash station 22. similar to the station 14, and is then dried at a drying station 24.

The strip 7 then passes to a powder coating station, shown generally at 26. Here the strip is sprayed with 0.3 percent solution of sodium or potassium silicate at a the rate of 2 to 5 cc/sq.ft.

A second layer of aluminum particles, e.g., of particle size 300 mesh/dust is applied in an electrostatically charged condition to each of the opposite sides 8, 9 of the moving strip 7, on top of the layer of iron thereon at a rate of say 10 grams per square foot, although this amount may be varied considerably if desired.

The strip 7 then passes through two drying stations 28, each incorporating a high frequency heater.

The strip 7 is then passed between the rolls of a rolling mill 30 to compact the aluminum powder thereto. If desired, a solution of sodium carboxy-methylcellulose may be applied before compaction so as to reduce the extent to which the said rolls might otherwise dislodge the aluminum powder. Thus there is a reduced tendency for banks of dislodged powder to build up immediately ahead of the rolls and thereby prevent the strip being moved at high speed.

The compacted strip 7 is then coiled onto two coilers 34 for subsequent heat treatment, the latter consisting in heating the coil in air for to hours at 502 to 600 C, and preferably in the range of 502 to 525 C. The compaction arising from the use of the said rolls merely produces a mechanical bond between the particles of aluminum powder and the electroplated iron layer, so that the particles of aluminum powder are merely mechanically secured in position without being sintered to each other and to the steel substrate. However, the said heat treatment causes the aluminum particles to be sintered and bonded to each other and to the steel substrate.

A shear 32 is provided so that the' apparatus can be operated continuously. When one coiler 34 is full, the shear 32 shears the strip 7 whilst it is moving at line speed. The strip 7 is then coiled on the other coiler 34 and the previously coiled strip 7 is removed for the said heat treatment.

Referring in more detail to the electrolytic deposition of the iron layer, we have found that iron can be electrolytically deposited from a wide variety of solutions. In common with other electroplated metals, bath additions, pH, temperature, and ion concentration, have an influence on the type of deposit obtained. We have chosen an aqueous solution of ferrous chloride which yields an adherent deposit whilst requiring little control. Such a bath is readily obtained from waste pickle liquor by neutralising with excess iron. Details of a suitable electroplating bath are:

Iron: 50 100 gms/litre; pH: 2 to 2.2; Temperature: 60 C Current: up to 500 Amp/sq. ft., normally 300 Amp/sq. ft.

A rather surprising fact has emerged from our work on electroplated iron. We have found that the steel can be pickled in hydrochloric or sulphuric acid prior to electroplating. Thus we do not experience to such a degree the problems of hydrogen evolution during sintering which would otherwise suggest the use of nitric acid, perhaps because the iron layer acts as a barrier which prevents hydrogen being introduced into the surface of the steel when in the pickling bath. In order to use hydrochloric or sulfuric acid in the pickling baths, a layer of iron at least 0.5 microns thick must be deposited by electrolysis in order to avoid blistering and bad adhesion of the coating after sintering, even at 300 C.

A coated strip having a layer of electroplated iron has a performance under thermal cycling dependent on the thickness of the layer of iron. This is shown by the following Table, which relates to -tests performed on a I complished or the material had failed.

Thickness of first layer Tempcr- No. of of iron ature, cycles to (microns) C failure Type of failure 25. 500 20 Large blisters on coating. 5. 502 No failure. .5. 525 100 Do.

5. 550 10 Few small blisters. O. 550 100 N0 failure. 5. 575 25 Isolated blisters. 0. 580 100 N0 failure. .5 600 100 Trace of blistering. 5. 600 100- No failure. 5... 625 100 Coating fully alloyed with substrate without any failure.

' Possibly due to porosity in iron layer.

It can'be seen from the Table that the coiled strip coiled on the coilers 34 can be sintered at 5 50 C., provided the layer of iron is at least 1 micron thick. We think that this is because the intermediate layer of alloydiscussed previously is not formed at 550 C and this temperature can safely be used for sintering. We have found that sintering at temperatures above 500 C yields a product with superior corrosion resistance to that sintered at lower temperatures.

We think that the provision of the electroplated layer of iron retards the nucleation of an iron/aluminum alloy layer by raising the temperature below which little or no nucleation occurs. We think that this temperature is raised due to the higher purity of the iron layer compared to the steel substrate. The steel is of course impure" iron in the sense that it has other constituents alloyed with iron.

When nucleation commences, the growth of the alloy from given nucleates is very rapid due to the fast rate of reaction at the raised nucleation temperature. As a result the risk of blistering and delamination is reduced. Thus the nucleation appears to be controlled such that the alloy grows in a toothy pattern into the aluminum layer, thereby promoting adhesion of the layers to each other.

Moreover, because of the said retardation of the growth of the alloy, sintering may be effected at a higher temperature than usual (i.e., at a temperature within the range 502 to 600 C for 15 to 20 hours) and this leads to better corrosion resistance. The preferred temperature range within this range is 502 to 525 C.

EXAMPLE n The method of this Example was carried out as described above in connection with Example-I except that between the wash station 18 and the wash station 22, the apparatus shown in FIG. 2 was employed in substitution for the electro-plating baths 20.

Thus in the case of Example II, the strip 7, after leaving the water wash station 18, passed over rolls 35 to 56, after which the strip 7 passed to the wash station 22. The rolls 38, 41, 44, 47, 50, 53 and 56 were conductor rolls which fed electrical current into the strip 7, the' remaining rolls being insulated. The rolls 36, 37 were disposed in a pre-dip tank 60. The rolls 39 and 40, 42 and 43, and 45 and 46 were respectively disposed in three iron plating baths 61, 62, 63 for electroplating the surface 9 of the strip 7, and the rolls 48 and .49, 51

and 52, and 54 and 55 being respectively disposed in three iron plating baths 64, 65, 66 respectively for plating the surface 8 of the strip 7.

The pre-dip tank 60 was provided (by means not shown) with a supply of electrolyte from the adjacent plating bath 61 since, in order to obtain good adhesion between the electro-plated iron layer and the steel substrate, it was found desirable to condition the strip by passing it through a solution at the same pH as the plating solution, and this could most easily be done by conditioning the strip by passing it through some of the plating solution itself.

As will be seen from FIG. 2, the strip 7 in passing through each of the baths 61 to 66 passed immediately above an iron anode 67 therein so that whichever surface of the strip 7 was lowermost in the respective bath, was plated with iron. In the baths 64, 65, 66, however, the surface 9 of the strip, which had been plated with iron as a result of its having been the lowermost surface when the strip was passing through the baths 61 to 63, was now, in passing through the tanks 64, 65, 66, the uppermost surface of the strip, and thus passed through the latter tanks without being cathodically polarised. Consequently, unless steps were taken to prevent this, the surface 9 could be attacked by the acidic electrolyte in the baths 64 to 66 and could thus, in effect, be picked in an acid solution which would introduce hydrogen into the substrate.

In order to overcomethis problem, each of the baths 64 to 66 was provided with an auxiliary anode 70 made of titanium, which was thus insoluble in the electrolyte.

. These auxiliary anodes 70 were operated at a very low voltage which was just sufficient to ensure that the surface 9 was cathodically polarised and could not therefore be pickled. It was found that this could be achieved by applying to the auxiliary anodes 70 a current density of about amps per square foot, which is very low when compared to the main plating current density of 250 to 500 amps per square foot.

, It might be thought that it would also be possible to overcome this problem by plating both sides of the strip 7 simultaneously. This can of course be done, but in that case there is some difficulty in supporting a top anode which may weigh about 3 tons, and difficulties also arise from the fact that an iron anode does not dissolve uniformly and that lumps of iron scale therefore drop onto the top of the strip. Consequently, it is preferred to use the arrangement shown in FIG. 2, in which one side of the strip is plated at a time and use is made of auxiliary anodes.

The iron content of the electrolyte in the baths 61 to 66 was at least 70 grams per litre as ferrous iron, since failure to maintain this concentration could cause blisters and poor adhesion in the final product. The plating solution was kept at a pH of 1.2 and was operated at a temperature of 75 C.

The strip 7, after leaving the 'roll 55 in the last plating bath 66, and before reaching the roll 56, passed between apair of air knives '71 which directed large volumes of low pressure air (e.g., at ambient temperature) onto the surfaces 8, 9 of the strip 7. The purpose water in that solution was evaporated. This step has been found to be of considerable importance, since it is otherwise difficult or impossible to obtain a clean, stain-free iron deposit if the strip 7 is passed into the water wash station 22 while it is still wet with ferrous chloride solution. Whilst it is not entirely clear as to why this is so, it should be noted that the ferrous chloride solution is acidic and can corrode the electroplated iron layer, and that the action of water on the acidic solution is to cause a rise in pH with subsequent precipitation of insoluble hydrated iron oxideonto the strip. The use of the air knives avoids such staining and thus ensures that a satisfactoryaluminum coated steel can be produced.

We claim:

1. A method of providing a surface of a steel substrate with an aluminum coating consisting essentially of the steps of electrolytically coating the said surface with a layer of iron, wetting the iron layer with a film of an aqueous solution consisting essentially of water and an alkali metal silicate, coating the said wetted layer of iron with a layer of aluminum powder which forms an alloy with the steel substrate when heat treated at a temperature of 502 600 C, compacting the said layers to the surface of the steel substrate at a temperature below the sintering temperature whereby the particles of aluminum powder in the compacted layer thereof are compacted to each other and to the steel substrate without sintering occurring, and heat treating the compacted steel substrate at a temperature of 502 600 C over a period of time whereby sintering and bonding of the particles to each other and to the steel substrate occurs, the said iron layer serving to control nucleation of said alloy during the heat treatment and having a thickness varying from a minimum of about 0.5 micron when the heat treatment is at 502 C to a minimum of about 3.5 microns when the heat treatment is at 600 C.

2. A method as claimed in claim 1 in which the heat treatment is in the range 502 575 C.

3. A method as claimed in claim 1 in which the heat treatment is in the range 502 525 C.

4. A method as claimed in claim 3 in which the thickness of the iron layer is at least 1.5 microns.

5. A method as claimed in claim 1 in which the compacted steel substrate is heat treated for' about 15 20 hours. 1

6. A method as claimed in claim 1 in which the aluminum powder particles are applied to the wetted iron layer in an electrostatically charged condition.

7. A method as claimed in claim 1 in which the substrate is a moving steel strip each of whose opposite sides is provided with an aluminum coating.

8. A method as claimed in claim 1 in which the surface of the steel substrate is subjected to pickling in an acid, the electrolyte employed during the said electrolytic coating being derived from liquor resulting from the said pickling.

9. A method as claimed in claim 8 in which any remaining acid in the pickle liquor is neutralised with scrap iron, and the pickle liquor is diluted with water prior to its being used as the electrolyte.

10. A method as claimed in claim 8 in which the acid is hydrochloric acid.

1 l. A method as claimed in claim 1 in which the steel substrate, immediately after receiving the electrolytic coating, is dried so as to solidify any electrolyte on the most, through at least one second plating bath, each of the first and second plating baths being provided with an iron anode above which the steel strip is passed and the said at least one second plating bath being provided with an auxiliary anode disposed above the steel strip, the auxiliary anode being insoluble in the electrolyte in the second plating bath and being operated at a low voltage which cathodically polarises the steel strip and ensures that the latter is not pickled by the said electro lyte.

14. A method as claimed in claim 13 in which the auxiliary anode is a titanium anode and is operated at a current density of about 5 amperes per square foot.

15. A method as claimed in claim 1 in which the electrolyte employed in the electrolytic coating has a ferrous iron content of at least 70 grams per litre.

16. A method as claimed in claim 1 in which the temperature of the electrolyte employed in the electrolytic coating is at least 70 C.

17. A method as claimed in claim 1 in which, immediately prior to the electrolytic coating, the steel substrate is wetted with a liquid whose pH is substantially the same as that of the electrolyte employed in the electrolytic coating.

18. A method as claimed in claim 17 in whichthe electrolyte itself is employed as the said liquid.

19. A method as claimed in claim 1 in which the alu- 

2. A method as claimed in claim 1 in which the heat treatment is in the range 502* - 575* C.
 3. A method as claimed in claim 1 in which the heat treatment is in the range 502* - 525* C.
 4. A method as claimed in claim 3 in which the thickness of the iron layer is at least 1.5 microns.
 5. A method as claimed in claim 1 in which the compacted steel substrate is heat treated for about 15 - 20 hours.
 6. A method as claimed in claim 1 in which the aluminum powder particles are applied to the wetted iron layer in an electrostatically charged condition.
 7. A method as claimed in claim 1 in which the substrate is a moving steel strip each of whose opposite sides is provided with an aluminum coating.
 8. A method as claimed in claim 1 in which the surface of the steel substrate is subjected to pickling in an acid, the electrolyte employed during the said electrolytic coating being derived from liquor resulting from the said pickling.
 9. A method as claimed in claim 8 in which any remaining acid in the pickle liquor is neutralised with scrap iron, and the pickle liquor is diluted with water prior to its being used as the electrolyte.
 10. A method as claimed in claim 8 in which the acid is hydrochloric acid.
 11. A method as claimed in claim 1 in which the steel substrate, immediately after receiving the electrolytic coating, is dried so as to solidify any electrolyte on the steel substrate, the solidifed electrolyte being thereafter removed by washing with water, and the steel substrate being then wetted with the said aqueous solution of an alkali metal silicate.
 12. A method as claimed in claim 11 in which the drying is effected by directing air onto the steel substrate.
 13. A method as claimed in claim 7 in which the steel strip is passed, with a predetermined surface thereof lowermost, through at least one first plating bath, and is then passed, with the opposite surface thereof lowermost, through at least one second plating bath, each of the first and second plating baths being provided with an iron anode above which the steel strip is passed and the said at least one second plating bath being provided with an auxiliary anode disposed above the steel strip, the auxiliary anode being insoluble in the electrolyte in the second plating bath and being operated at a low voltage which cathodically polarises the steel strip and ensures that the latter is not pickled by the said electrolyte.
 14. A method as claimed in claim 13 in which the auxiliary anode is a titanium anode and is operated at a current density of about 5 amperes per square foot.
 15. A method as claimed in claim 1 in which the electrolyte employed in the electrolytic coating has a ferrous iron content of at least 70 grams per litre.
 16. A method as claimed in claim 1 in which the temperature of the electrolyte employed in the electrolytic coating is at least 70* C.
 17. A method as claimed in claim 1 in which, immediately prior to the electrolytic coating, the steel substrate is wetted with a liquid whose pH is substantially the same as that of the electrolyte employed in the electrolytic coating.
 18. A method as claimed in claim 17 in which the electrolyte itself is employed as the said liquid.
 19. A method as claimed in claim 1 in which the aluminum coated steel substrate is cold reduced and then annealed at a temperature within the range 500* - 525* C. 